Temperature gradient zone melting apparatus

ABSTRACT

An apparatus is provided for fabricating a semiconductor device by thermal gradient zone melting, whereby metal-rich droplets such as aluminum migrate through a semiconductor wafer such as silicon to create conductive paths. One surface of the wafer is placed directly on a heating surface to establish a high and uniform thermal gradient through the wafer. Heat in the wafer is removed from the other wafer surface. The apparatus for fabricating semiconductor devices utilizing temperature gradient zone melting comprises a base, heating means and heat sink means. Heating means comprises a platform having a generally planar heating surface adapted to receive the entire area of the one surface of at least one wafer. The heat sink means is spaced away from the other wafer surface to form a space therebetween, the space being adapted to receive a high heat conductive gas. The heat sink means and the gas cooperatively remove the heat in the wafer to enhance the establishment of the thermal gradient.

TECHNICAL FIELD

This invention relates to processes for fabricating semiconductordevices and, more particularly, to processes using the temperaturegradient zone melting technique for fabricating three-dimensionalmicroelectronic devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

A related process employing a buffer layer is described and claimed in arelated application, formerly U.S. Ser. No. 366,900, now patented asU.S. Pat. No. 4,398,974, by Kuen Chow and Jan Grinberg. The relatedpatent discloses and claims a process comprising the step of applying abuffer layer to a wafer for efficacious fabrication of semiconductordevices. The present application and the related application nowpatented as U.S. Pat. No. 4,398,974, are assigned to a common assignee.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Demands for ever faster transfer of signals in digital computers haveled to the advent of three-dimensionally structured microelectronicsemiconductor devices for use in such computers. One suchthree-dimensionally structured device is disclosed in U.S. Pat. No.4,275,410, by Grinberg et al. In that device, signals are transferredthrough stacked semiconductor wafers each of which is interconnectedwith adjacent wafers by external leads extending from both surfaces ofthe wafer. Transfer of signals through each wafer is provided byinternal connections which are positioned between the wafer surfaces.The process for fabricating such internal connections is the subject ofthe present invention.

2. Description of the Prior Art

One process for fabricating such internal connections is a techniquegenerally called temperature gradient zone melting; an example of whichis disclosed in U.S. Pat. No. 3,897,277, by Blumenfeld. In this process,a liquid zone migrates in a solid along a thermal gradient. Inparticular, a liquid droplet of metallic material placed on one surfaceof a solid wafer migrates through the solid and emerges on the wafer'ssecond surface. The wafer is heated to an elevated temperature forestablishing a thermal gradient through the wafer, the hottertemperature being at the second surface. The thermal gradient causes thetemperature at the forward interface of the droplet to be higher thanthe temperature at its rear interface. Since the solubility of a solidin a liquid increases with temperature, the concentration of the atomsfrom the dissolved wafer solid is greater at the hotter forwardinterface of the liquid than at the cooler rear interface. Theinequality in concentration produces a concentration gradient of atomsfrom the dissolved wafer solid across the liquid droplet. Theconcentration gradient, in turn, creates a flux of atoms from thedissolved wafer solid along this gradient, that is, a flow of atoms fromthe hotter front interface to the cooler rear interface. To feed thisdiffusion flux, additional atoms of the wafer solid are dissolved intothe liquid at the forward interface and moved to the rear interface.Consequently, the liquid droplet migrates along the thermal gradienttoward the hotter second surface, dissolving atoms of the wafer at itsforward interface, passing the atoms toward the rear, and redepositingthese atoms at its rear interface. At the cooler rear interface, theredeposited atoms recrystallize with traces of the droplet metallicmaterial in them. Thus, the path left by the migrating liquid droplet ishigher in conductivity than rest of the wafer. The conductive path,extending from one wafer surface to another, is the internal connectionnecessary for the operation of a three-dimensional microelectronicdevice. However, prior art processes employing the temperature gradientzone melting technique are deficient in several aspects.

One deficiency in the prior art is the inability of the prior artprocesses to produce a high thermal gradient through the wafer. Sincethe thermal gradient in a wafer is generally proportional to thequantity of heat that flows through the wafer and correspondingly themigration rate of the droplets is generally proportional to the thermalgradient, the quantity of heat flowing through the wafer should be ashigh as possible. However, both the duration that the wafer is exposedto the heat and the quantity of heat itself invariably cause defects inthe wafer. Thus, creating a high thermal gradient through a waferwithout using an even greater quantity of heat would result in the twinbenefits of faster migration rate and less exposure to heat, producingmore defect-free wafers.

One prior art process of enhancing the thermal gradient through a waferis disclosed in U.S. Pat. No. 4,033,786, by Anthony et al., in whichanti-reflection coatings are applied to wafer surfaces for trapping heatin the wafer. However, this and other prior art processes are still notefficient because each of them provides a gas-filled gap between thewafer and the heating source. The gas usually acts as an insulator inpreventing the most efficacious transfer of heat from the heat source tothe wafer, thereby requiring either more heat to flow through the waferor longer exposure to heat in order to create the necessary thermalgradient. Examples of such processes are disclosed in U.S. Pat. No.3,895,967, by Anthony et al. and U.S. Pat. No. 3,910,801, by Cline etal.

Another deficiency in the prior art is the inability of the prior artprocesses to provide a uniform temperature gradient through the wafer,thereby causing nonuniform droplet migration. Uniformity refers to thecapability of providing both parallel heat flow lines for parallelmigration of droplets and linear isotherms parallel to the wafersurfaces which indicate the establishment of the same temperature at agiven depth in the wafer. Uniform droplet migration is essential inorder to maximize the number of completed migrations within a certainprocess time limit, a completed migration being a generally verticalinternal connection with exposed ends on both surfaces of the wafer.Without such uniformity, the yield of usable conductive internalconnections in each wafer decreases.

The parallel heat flow line type of nonuniformity is usually caused bysupport pins or holders that are necessary for supporting the wafer inthe heating apparatus. The pins or holders, acting like heat sinksbecause they are more heat conductive than the surrounding gas, createdeviations in the parallel heat flow lines through the wafer. Examplesof the use of such pins and holders are described in the processesdisclosed in U.S. Pat. No. 3,895,967, by Anthony et al. and U.S. Pat.No. 4,001,047, by Boah. To alleviate this problem, support ribs andguard rings around the periphery of the wafer are used to preventlateral heat flow. Such devices are used in processes disclosed in U.S.Pat. No. 3,895,967, by Anthony et al. and U.S. Pat. No. 4,035,199, byAnthony et al. respectively.

The linear isotherm type of nonuniformity occurs in infrared heatingprocesses such as the process disclosed in T. R. Anthony and H. E.Cline, "Stresses Generated by the Thermomigation of Liquid Inclusions inSilicon," Journal of Applied Physics, Vol. 49, No. 12, December, 1978.Because silicon is semi-transparent to infrared radiation, infraredradiation is absorbed at varying depths in the wafer. The absorbedradiation, thus, provides a variety of temperatures at a given depth ofthe wafer, creating nonlinear isotherms through the wafer. In turn, thenonlinear isotherms created by an infrared process cause non-uniformmigration of the droplets. In contrast, non-infrared processes providethe same temperature at a given depth of the wafer, thereby creatinglinear isotherms which are parallel to the wafer surfaces. Aluminumdroplets therefore, migrate through a uniform distance of the wafer froma lower-temperatured isotherm to a higher-temperatured isotherm duringthe same time period.

The prior art processes, thus, are deficient in their inability toproduce a high thermal gradient through the wafer and to provide auniform temperature gradient through the wafer.

SUMMARY OF THE INVENTION

In view of the deficiencies in the prior art, it is a purpose of thepresent invention to provide a temperature gradient zone melting processand an apparatus capable of providing a high thermal gradient through awafer.

It is another purpose of the present invention to provide a temperaturegradient zone melting process and an apparatus capable of providing auniform thermal gradient through a wafer.

In order to accomplish the above and further purposes, the presentinvention provides a novel process for fabricating a semiconductordevice by thermal gradient zone melting and a novel apparatus forpracticing the novel process. The process comprises the steps of firstselecting a wafer body of suitable semiconductor material which is ofone type of conductivity. The wafer body has two major generallyparallel opposed surfaces, the opposed surfaces being respectively thetop and bottom surfaces. Next, an array of droplets in solid form isdeposited on the top surface, the droplets comprising suitablemetal-rich conductive material which is of an opposite type ofconductivity. In the art, droplets are generally referred to as suchirrespective of either solid or liquid form. The wafer body is thenplaced on a surface of heating means, with the entire area of the bottomsurface in direct physical contact with the heating surface.

The wafer body is heated to a sufficient temperature to establish athermal gradient through the wafer body wherein the bottom surface,which is in direct contact with the heating surface, is at a highertemperature than the top surface. Simultaneously, the droplets areheated to a liquid state to enable their migration through the waferbody toward the heating surface. Moreover, heat in the wafer body isremoved through its top surface in order to enhance the establishment ofthe thermal gradient. Thus, the droplets form column-like paths each ofwhich has a conductivity opposite to that of the wafer body.

The apparatus for practicing the novel process comprises a base, heatingmeans, and heat sink means. The heat means, which is mounted on thebase, comprises a platform having a generally planar heating surfacewhich is adapted to receive the entire area of the bottom surface of atleast one wafer body. The heat sink means is spaced away from the topsurface to form a space therebetween, the space being adapted to receivea high heat conductive gas. The heat sink means and the gascooperatively remove heat in the wafer body from its top surface toenhance the establishment of the desired thermal gradient.

One advantage of the present invention is that a high thermal gradientis established through the wafer. Such a high gradient results from thecombination of the direct placement of the wafer on the heating surfaceand the efficacious cooling of the wafer by the cooperative action ofboth the heat sink means and the heat conductive gas. With the bottomsurface of the wafer in direct physical contact with the heating surfaceand the top surface in close proximity with the heat sink means, thedesired large heat flow through the wafer is created, which in turncauses the high thermal gradient. Having the high thermal gradient andthe attendant faster migration rate, either the heat flowing through thewafer may be lowered or the manufacturing time may be shortened. Eitherresult produces more defect-free wafers.

Another advantage of the present invention is that a uniform thermalgradient is established through the wafer. The uniformity results fromthe direct placement of the wafer on the heating surface. The highthermal gradient enhances parallel heat flows and eliminates lateralheat flows in order to provide substantially parallel migration ofdroplets. In addition, the direct conduction of heat through the wafer,as contrasted to infrared heating processes, creates uniform linearisotherms, which are parallel to the wafer surfaces. This enhancesuniform droplet migration.

Other purposes, features, and advantages of the present invention willappear from the following detailed description of a preferred embodimentthereof, taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of stacked semiconductor wafers, theprocess for making such wafers being the subject of the presentinvention;

FIG. 2 is a simplified cross-sectional view of a novel apparatus usefulin practicing the novel process of the present invention;

FIG. 3 is a partial enlarged view of the apparatus of FIG. 2, depictingthe treatment of one partially cross-sectioned wafer;

FIGS. 4 to 7 are simplified sequential cross-sectional views of themigration of one droplet through a wafer in the process of the presentinvention; and

FIG. 8 is an enlarged simplified cross-sectional view of FIG. 3, with abuffer layer provided between the wafer bottom surface and the heatingsurface.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is shown a three-dimensional microelectronicdevice 12 which comprises a plurality of stacked semiconductor wafers14. As an example, three of such wafers 14 are shown. Each wafer 14 isconnected to another wafer 14 by external leads 16 which may extend fromeither of surfaces 18, 20 of wafer 14, thereby allowing the transfer ofsignals from one wafer 14 to another. The transfer of signals throughwafer 14 is provided by internal connections 22 which are positionedbetween wafer surfaces 18, 20. The process for fabricating such internalconnections 22 is the subject of the present invention.

The process of the present invention employs a technique generallyreferred to as thermal gradient zone melting. As best illustrated in thesequence of FIGS. 4 to 7, a wafer 14 of suitable semiconductor material,which is of one type of conductivity, is selected. For example, a wafer14 of n-type silicon is selected. Wafer 14 has a top surface 18 and abottom surface 20. Solid droplets 24, a suitable metal-rich conductivematerial of an opposite type of conductivity, are placed on wafer topsurface 18, as best shown by a representative single droplet 24 in FIG.5. As an example, droplets 24 of aluminum are selected. In the art,droplets are generally referred to as droplets irrespective of eithersolid or liquid form. Moreover, droplets 24 may be deposited in apredetermined pattern, as depicted by FIG. 1.

Wafer 14 is then heated to an elevated temperature in order to establisha thermal gradient through wafer 14, with bottom surface 20 being thehotter surface. Simultaneously, aluminum droplet 24 is heated to form aliquid which is generally referred to as a liquid zone. As shown inFIGS. 6 and 7, droplet 24 then migrates through wafer 14 toward hottersurface 20. As it migrates, droplet 24 leaves a recrystallized pathwhich is of an opposite conductivity than the conductivity of wafer 14.The column-like path, p-type silicon in the example, eventually becomesconductive connection 22.

Referring now to FIG. 2, there is shown a novel apparatus 30 useful inpracticing the novel process of the present invention. Apparatus 30comprises removable chamber wall 32 and chamber base 34 which combine toform thermomigration chamber 36. Within chamber 36, apparatus 30 furthercomprises rotatable heating platform 40 which includes a generallyplanar heating surface 42 for supporting at least one wafer 14, two ofwhich are shown as an example in FIG. 2. As best shown in FIG. 3,heating surface 42 is adapted to receive the entire area of wafer bottomsurface 20. Platform 40 generally comprises a heat conductive material,for example, graphite. Radio-frequency coils 44 are positioned beneathheating platform 40 to induce heat therein in order to heat wafers 14. Aheat sink 46, which is spaced from heating surface 42, is provided forwafers 14. In the example, heat sink 46 is a water tank. In addition,gap 48 between wafer top surface 18 and water tank 46, as best shown inFIG. 3, is provided to receive a gas of high heat conductivity such ashydrogen or helium. The gas may be in either a pure or a mixed form. Inthe example, helium is selected. The cooperative action of water tank 46and the gas removes heat from wafer 14 to enhance the establishment ofthe desired thermal gradient through wafer 14.

Furthermore, apparatus 30 comprises radio-frequency conductors 50 which,exiting through chamber base 34, are connected to a radio frequencysource, not shown. Heating platform 40 is mounted on rotatable platformdrive shaft 60. Motor 64, which is connected to elevatable drive shaft62 which in turn is connected to platform drive shaft 60, is adapted torotate platform 40. Gas conduit 57, connected to gas entry port 56 atits external end 61, is provided within platform drive shaft 60 anddrive shaft 62. Internal end 63 of gas conduit 57 is positioned adjacentheating platform 40. In addition, water entry and exit ports 52, 54, andgas entry and exit ports 56, 58 are provided.

In the preferred practice of this invention, a wafer 14 of one type ofconductivity, for example, n-type silicon, is selected. Wafer 14 has twomajor parallel opposed top and bottom surfaces 18, 20, as best shown inFIG. 3. Wafer 14 of the preferred embodiment has a thickness, designatedas d_(W), of typically 20 mils. Next, an array of solid droplets 24 ofsuitable metal-rich conductive material is placed on top surface 18 ofwafer 14, as best shown by the representative single droplet 24 in FIG.5. Droplets 24 are of an opposite conductivity, for example, aluminum.This array, arranged in a typical 32×32 matrix as similar to what isshown in FIG. 1, is applied to surface 18 by conventional techniquessuch as direct evaporation using a shadow mask. Each droplet 24 in thepreferred embodiment, for example, is approximately 3 mils in diameterand 8 to 25 micrometers in height, with a droplet center-to-dropletcenter distance of approximately 20 mils.

In the preferred process, two wafers 14, as an example, are placeddirectly on graphite heating platform 40, with the entire area of waferbottom surface 20 in direct physical contact with heating surface 42, asbest shown by the single wafer 14 in FIG. 3. Referring to FIG. 2, RFcoils 44 then induce eddy currents in graphite platform 40 which, beingresistive in property, transforms electrical energy into heat. Coils 44receive RF energy through conductors 50 which in turn are connected to aRF source, not shown. Simultaneously, the heat conductive gas enterschamber 36 through entry port 56 and water enters water tank 46 throughentry port 52. Gap 48, d_(G), which is approximately 5 mils in distancein the example, is provided to receive the gas. As wafer bottom surface20 is heated by platform 40 and as wafer top surface 18 is cooled by thecooperative action of both the gas and water tank 46, a high thermalgradient through wafer 14 is established. The gas in gap 48, being aneffective heat conductive gas, removes the heat in wafer 14 from topsurface 18 and transports the heat to water tank 46 for cooling.

With the entire wafer bottom surface 20 in direct physical contact withheating surface 42, a uniform thermal gradient is also established,thereby enhancing the number of completed droplet migrations. Heat inwafer 14 generally flows in a parallel fashion, as best shown by theheat flow lines 26 in FIGS. 4 to 7, for enhancing uniform dropletmigration. In addition, direct heating of wafer 14 enhances theestablishment of linear isotherms 28 for uniform migration of droplets24, as best shown in FIG. 3. Linear isotherms 28 are parallel to wafersurfaces 18 and 20, as best shown in FIGS. 4 to 7. Rotation of platform40 by motor 64 also provides a uniformity of heat at heating surface 42for uniform heating of wafer bottom surface 20. Furthermore, forexample, water of room temperature in tank 46, exiting through port 54,conveniently circulates at a rate of approximately 5-6 gallons perminute for cooling the heat conductive gas. Similarly, for example, thegas in chamber 36, exiting through port 58, conveniently circulates at arate of approximately 10 cubic feet per minute for transporting the heatfrom wafer 14 to water tank 46.

Simultaneous with the establishment of the thermal gradient throughwafer 14, droplets 24 are heated to a liquid state. With a high thermalgradient established through wafer 14, liquid aluminum droplets 24migrate uniformally toward heating surface 42, as partially shown inFIG. 3. As droplets 24 migrate through wafer 14, they leaverecrystallized connecting paths 22 which are of an opposite conductivityto wafer 14, as best shown in FIGS. 3, 6 and 7. For example, paths 22are p-type silicon which contain traces of aluminum. Typically, each ofthe resultant internal connections 22 has a resistance of approximately5 to 15 ohms.

In the preferred embodiment, a buffer layer 70 is applied to the entirearea of bottom surface 20, interposing between bottom surface 20 andheating surface 42. Buffer layer 70 has the characteristic of beinginsoluble with liquid droplets 24. This enables buffer layer 70 to trapand terminate the migration of droplets 24, thereby preventing alloyingor sticking of droplets 24 with heating surface 42. Buffer layer 70,thus, facilitates the subsequent removal of wafer 14 from heatingsurface 42. Buffer layer 70 comprises a material such as silicon dioxideor aluminum oxide. The process employing buffer layer 70 is describedand claimed in the above-mentioned copending application by Kuen Chowand Jan Grinberg.

Buffer layer 70, which is silicon dioxide of conventional purity in theexample, is applied to wafer bottom surface 20 by conventionaltechniques such as vapor deposition. Buffer layer 70, for example, isapproximately one micrometer in thickness. After removal from heatingsurface 42, buffer layer 70 is removed by conventional resurfacingtechniques such as grinding and polishing.

With the heat flowing through wafer 14 at approximately the typicaltemperature of 1000° C., a thermal gradient of 250° C./cm is establishedthrough wafer 14 in accordance with the invention. The resultant thermalgradient is approximately 2 to 5 times larger than the gradientsdisclosed in the prior art. The comparatively high thermal gradientallows the migration of droplets 24 to be completed in approximately 10minutes. With wafers 14 being exposed to chamber heat for only such acomparatively short duration, more defect-free wafers are manufactured.Moreover, such a comparatively high thermal gradient enhances uniformmigration of droplets 24, which in turn produces more wafers withcompleted migrations.

It will be apparent to those skilled in the art that variousmodifications may be made within the spirit of the invention and thescope of the appended claims. For example, the distance of gap 48,d_(G), may be narrowed. Because the thermal gradient generallycorresponds inversely to the distance of gap 48, a narrowing of distanced_(g) to 2 mils increases the thermal gradient across wafer 14. This isdue to the fact that water tank 46 is now closer to wafer 14 and is ableto provide more heat removing capability. The narrowing of d_(G) isaccomplished by elevating drive shaft 62, which in turn elevatesplatform 40 and wafer 14 supported thereon. The narrowing of d_(G)allows the process to proceed at either an even faster migration rate ora lower temperature. The lower temperature process permits migrations ina lower thermal gradient. Either selection results in even moredefect-free wafers.

What is claimed is:
 1. An apparatus for fabricating a semiconductorwafer by thermal gradient zone melting, said wafer body having generallyparallel opposed top and bottom surfaces, said apparatus comprising:(a)a base; (b) a rotatable heating means mounted on said base, said heatingmeans having a generally planar heating surface which is capable ofsupporting the entire area of the bottom surface of the semiconductorwafer, which aids in establishing a substantially uniform thermaltemperature gradient through said wafer such that said bottom surface ofsaid wafer is maintained at a higher temperature than said top surface;(c) a source of heat conductive gas; (d) heat sink means sufficientlyspaced from the heating means but in close proximity therewith, suchthat a semiconductor wafer placed on said heating means forms a minimalspace between said heating means and said heat sink means, said spacecapable of receiving a high heat conductive gas, such that said heatsink means and said gas cooperatively remove heat from within said waferbody through the top surface of said semiconductor wafer to enhance theestablishment of a thermal temperature gradient; and (e) means forproviding entry and exit of said heat conductive gas to and from saidspace.
 2. The apparatus as claimed in claim 1 further comprising:anupper wall adapted to be removably mounted on said base, therebydefining an enclosed chamber containing said heating means, said heatsink means, and said gas space therein.
 3. The apparatus as claimed inclaim 2 further comprising:gas entry and exit ports connected to saidchamber adapted to provide said gas to said chamber.
 4. The apparatus asclaimed in claim 1 or 2, whereinsaid heat sink means comprises a watertank having cooling water therein.
 5. The apparatus as claimed in claim1 or 2, whereinsaid heating platform is graphite.
 6. The apparatus asclaimed in claim 5, whereinsaid heating means comprises radio-frequencycoils positioned adjacent said graphite platform for inducingheat-generating currents in said platform.